Make FirstSelectableOption() and LastSelectedOption() of HTMLSelectElement faster

Resolves two TODO()s.
This CL has no behavior changes.

Change-Id: Ibee5cc16246703940b3312fcbea05a9b2a529e51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066420Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743418}

third_party/blink/renderer/core/html/forms/html_select_element.cc

@@ -537,15 +537,11 @@ HTMLOptionElement* HTMLSelectElement::PreviousSelectableOption(
 }
 
 HTMLOptionElement* HTMLSelectElement::FirstSelectableOption() const {
-  // TODO(tkent): This is not efficient.  nextSlectableOption(nullptr) is
-  // faster.
-  return NextValidOption(GetListItems().size(), kSkipBackwards, INT_MAX);
+  return NextValidOption(-1, kSkipForwards, 1);
 }
 
 HTMLOptionElement* HTMLSelectElement::LastSelectableOption() const {
-  // TODO(tkent): This is not efficient.  previousSlectableOption(nullptr) is
-  // faster.
-  return NextValidOption(-1, kSkipForwards, INT_MAX);
+  return NextValidOption(GetListItems().size(), kSkipBackwards, 1);
 }
 
 // Returns the index of the next valid item one page away from |startIndex| in